1. Field of the Invention
The present invention relates to a vacuum drier apparatus for drying materials such as heat sensitive chemical materials and powders, for example.
2. Description of the Background Art
A vacuum drier apparatus has been used for drying materials such as heat sensitive chemical materials and powders, for example.
Such a conventional vacuum drier apparatus is usually equipped with an openable door having an observation window made from a piece of tempered glass plate. This tempered glass plate is designed to have such a thickness that it can endure the atmospheric pressure, and may be shaped in a protruding form.
However, it is necessary to provide a protection against an accidental breaking of this tempered glass plate due to an exertion of external forces. Such a protection is usually provided in a form of a protection plate such as that made of metallic mesh or transparent plastic plate which is placed in front of the tempered glass plate. This protection plate also serves, in a case of an accidental breaking of the tempered glass plate, to prevent the scattering of the broken glass pieces which are sucked into a vacuum drying chamber because of the reduction of pressure inside the vacuum drying chamber, and then bounce off a backside wall of the vacuum drying chamber.
For such a protection plate, the plastic plate is more commonly employed than a metallic mesh because it becomes easier to observe the inside of the vacuum drying chamber.
However, the protection plate made of a plastic plate also has a problem that it is subject to a thermal deformation due to the heat inside the vacuum drying chamber.
Also, conventionally, the protection plate made of a plastic plate has a problem in its strength, such that it sometimes does not function sufficiently as the protection plate because of its inadequate strength.
In a case when the protection plate made of a plastic plate and is attached with a slight clearance for entering the air from outside in order to prevent the reduction of pressure inside, there is a problem of contamination of the interior due to the external objects entering along with the air.